Ion implantation through resist-coated wafers is employed to control the doping levels in integrated circuit fabrication. The number of photoresist cleaning or stripping steps employed in the front end of the line (FEOL) semiconductor manufacturing process has grown greatly in the last few years. The increasing number of ion implantation steps needed in the device manufacturing process has driven this increase. Current high-current or high-energy implant operations (high dose implantation or HDI) are the most demanding in that they require a high degree of wafer cleanliness to be obtained while minimizing or eliminating photoresist popping, surface residues, and metal contamination, while requiring substantially no substrate/junction loss, or oxide loss.
The reason that HDI presents a particularly challenging FEOL cleaning operation is due to the physical structure of the post-implanted resist which becomes hardened and difficult to remove, as it must be before patterning operational steps are undertaken, where a different photoresist is applied for line patterning. The ion implantation process effectively depletes the surface of the resist of hydrogen and impregnates the resist film with the implant species. As a result of the implant species penetrating into the resist, the resist is transformed into two distinct layers, an outer, hardened, carbonized crust layer and an inner, bulk hydrogenated layer. Because the outer, carbonized layer has been essentially depleted of hydrogen, it can ash about 75% more slowly than the underlying inner, bulk photoresist layer. Additionally, this inner, bulk layer contains relatively high levels of chemically bonded nitrogen and marginal levels of the original casting solvent, which rapidly outgasses and expands when subjected to elevated ashing temperatures. This phenomenon causes photoresist popping as the outer, carbonized crust layer expands at a much slower rate than the underlying volatilized solvents causing the crust to rupture or “pop”. Photoresist popping is probably the greatest source of process defects with a plasma ashing system. The effects of the popping residues are a relatively high level of particles on implanted wafers, highly oxidized surface residues requiring aggressive post-ash wet cleans, and more frequent chamber cleaning and preventive maintenance procedures. Moreover, as residues accumulate in the process chamber, the probability of particles re-depositing on other wafers also increases. Another factor is that HDI can sputter silicon or silicon dioxide from the wafer substrate and deposit residues on top of the resist.
Therefore, following the ion implantation step(s) the resist and unwanted residues should be completely removed leaving the wafer surface residue-free, otherwise ineffective residue removal has the potential for high levels of process defects, and the quality of the cleaning step can directly effect electrical yield. Dry ashing followed by wet chemistry washing, e.g., oxygen plasma and a piranha wet-clean application, a mixture of sulfuric acid with either hydrogen peroxide or ozone, has generally been used to remove the hardened resist and residues. This process is costly and hazardous and also does not effectively remove inorganic residues, such as implant species, silicon, silicon dioxide and resist additives. Additionally, further wet chemistries are then required to remove these inorganic residues. Moreover, such dry ashing followed by those wet chemistry cleans causes unwanted damage to the doped wafers, i.e., to the source and drain areas of the doped wafer. Therefore, there is, a need for FEOL cleaning compositions that can effectively and efficiently strip-clean photoresist and ion implantation residues from ion implanted wafers, and for such strip-cleaning compositions that do not cause corrosion, i.e., alteration of the wafer structure in regard to the source and drain areas of the doped wafer.